In these days of energy and resource economization, there are eagerly demanded low cost, high performance carbon fibers used for the production of light weight comprise materials having a high tensile strength and a high modulus of elasticity required of aircrafts, motorcars, etc. and also molding carbon materials having a high tensile strength and a high density to be compression-molded to form various articles.
The compositions and structures of optically anisotropic pitches suitable for the production of high performance carbon fibers have not fully been elucidated. Further, a relationship between physical properties of carbonaceous pitches and the structures of compositions thereof has been unclear. There has not yet been completed a technique of stably controlling them on an industrial scale.
In optically anisotropic pitches heretofore disclosed such as those disclosed in the specifications of Japanese Patent Laid-Open Nos. 19127/1974 and 89635/1975, the optically anisotropic phase corresponds substantially to quinoline-insoluble portion (or pyridine-insoluble portion). As the optically anisotropic phase is increased closely to 100%, a softening point thereof is elevated remarkably and the spinning temperature is also elevated to approximately 400.degree. C. or higher, whereby a decomposed gas is formed from the pitch and the polymerization is caused during the spinning operation. Therefore, in the conventional carbon fiber spinning processes, the optically anisotropic phase content is controlled to up to 90% (practically, in the range of 50-65%) and the spinning temperature is controlled to a point at which the thermal decomposition or the thermal polymerization hardly occurs.
However, such a pitch composition is heterogeneous, since it comprises a mixture of an optically anisotropic phase and a considerable content of an optically isotropic phase. Accordingly, it has disadvantages that the fibers are broken during the spinning and the fibers have irregular thicknesses and a low tensile strength.
A pitch disclosed in the specification of Japanese Patent Publication No. 8634/1974 consists of seemingly substantially 100% optically anisotropic phase. This is a special pitch wherein the pitch molecules have limited, specific chemical structures. This pitch is prepared by the thermal polymerization of expensive pure substances such as chrysene, phenanthrene and tetrabenzophenazine and, therefore, constituents thereof have considerably controlled molecular weights. On the other hand, pitches produced from general mixed materials have quite high softening points. A pitch disclosed in the specification of Japanese Patent Publication No. 7533/1978 as a material for the production of carbon fibers has a low softening point and a low spinning temperature and is easily spun but the specification is silent on the optically antisotropic phase content. In said invention, the starting hydrocarbon is polycondensed in the presence of a Lewis acid catalyst such as aluminum chloride, the resulting pitch has specific composition and structure and carbon fibers produce from the pitch have insufficient tensile strength and modulus of elasticity. Said invention has another problem that the complete removal of the catalyst is difficult.
A pitch disclosed in the specification of Japanese Patent Laid-Open No. 55625/1979 is a homogeneous pitch consisting of essentially completely 100% optically anisotropic phase. However, it has a relatively high softening point in spite of narrow molecular weight distribution. In addition, said pitch has a low content of an n-heptane-soluble component (hereinafter referred to as component O) and a low content of an n-heptane-insoluble and benzene-soluble component (hereinafter referred to as component A) as will be described below in detail. Further, quinoline-insoluble component (hereinafter referred to as component C) in the balance of benzene-insoluble component is a large moiety of pitch. Therefore, the conventional pitch has a softening point of higher than about 330.degree. C. and a spinning temperature thereof is as high as 370.degree.-400.degree. C. In this temperature range, it is difficult to spin the pitch stably in an industrial basis.
As described above, known optically anisotropic pitches consisting of nearly 100% optical anisotropic phase have high softening points and they cannot be spun stably. On the other hand, pitches having low softening points (Except those produced from specific starting materials and having specific structures) are heterogeneous and they cannot be spun easily. Thus, it has been difficult to obtain carbon fibers having an excellent crystalloids.